Monolithic bone intended for implantation is treated in order to conserve its mechanical strength during lyophilization and subsequent packaging and to maintain the strength of the bone during the storage period preceding the rehydration and implantation of the bone.
The use of preserved bone intended for implantation to replace diseased or missing parts is common. The successful application of such bone is predicated on sound knowledge of its biologic properties and its capacity to withstand the stresses to which it will be subjected. When mineralized bone is used in grafts, it is primarily because of its inherent strength, i.e., its load bearing ability at the recipient site. The biomechanical properties of bone grafts upon implantation are determined by many factors, including the specific site from which the bone is taken; the age, sex, and physical characteristics of the donor; and the method chosen to prepare, preserve, and store the bone prior to implantation. A more detailed explanation of the alteration of the biomechanical properties of bone by the methods chosen for its preservation and storage may be found in Pelker et al., Clin. Orthop. Rel. Res., 174:54-57(1983). However, the needs for processing (e.g., to preserve the graft for later use and to remove immunogenic cellular materials) can conflict with the need to conserve the mechanical strength of the bone. During the preparation of bone intended for implantation the porous matrix is typically contacted with one or more treatment fluids to variously clean, defat, sterilize, virally inactivate, disinfect, and/or demineralize the bone or to impregnate the bone with one or more pharmacological agents (antibiotics, bone growth factors, etc.) so the bone can act as a drug delivery system. See U.S. Pat. No. 5,846,484 for a detailed explanation of the treatment of bone intended for implantation. Some treatment processes, such as irradiation and lyophilization, can work against conservation of the mechanical strength of bone and can lessen the bones weight bearing properties.
It is generally accepted that freezing monolithic bone to temperatures as cold as -70.degree. C. prior to its packaging and storage results in little if any alteration in its physical properties. However, freezing bone as a preservation technique is costly and can be logistically difficult, e.g., shipping and storage. Lyophilization (freeze-drying) is commonly performed on bone to permit its shelf storage for up to several years without spoilage. Lyophilization removes excess moisture from the bone and reduces its antigenicity. According to The American Association of Tissue Banks, lyophilized whole bone containing no more than 6% moisture can be stored at ambient temperatures for up to five years after processing. However, adverse changes in the biomechanical properties of the bone have been found to result from the lyophilization procedure. Lyophilization can result in damage to the bone due to dimensional changes that occur during the freezing and drying operations. The adverse mechanical changes appear to be associated with damage occurring in the bone matrix, specifically, ultrastructural cracks along the collagen fibers. These effects appear to be magnified when lyophilization and gamma irradiation are used together. Studies using rat bones to model the effects of lyophilization upon the compressive properties of cancellous bone (compression strength of tail vertebrae) and the bending and torsional properties of the long bones indicate that compressive strength can be reduced by up to 30% with little or no change in stiffness, bending strength can be reduced by as much as 40%, and torsional strength can be reduced by up to 60%. These changes have been found to occur even after the bone has been rehydrated. A more detailed explanation of the effects of lyophilization on mineralized bone can be found in Kang et al., Yonsei Med J 36:332-335(1995), and Pelker et al., J. Orthop. Res. 1:405-411(1984). Because freezing and thawing bone is minimally damaging to the bone, whereas lyophilization results in reduction in the mechanical strength of the bone, it is the inventors' belief that the mechanical strength-conserving agent is not acting as a cryopreservative (i.e., minimizing crystal growth during freezing) but rather in some new, not entirely understood, manner to diminish the dimensional changes associated with lyophilization.
Thus, it is desirable to provide a method for treating bone which is to undergo lyophilization as a prelude to its packaging and storage that will better conserve the biomechanical properties of the bone, i.e., its mechanical strength, as compared to untreated lyophilized bone, from the time the bone is harvested through the packaging and storage operations and to time of implantation.